Empirical data on variety, frequency, and prerequisites for unwanted side effects of psychotherapies are important regarding the planning, realization, and quality assurance of treatment. The study describes adverse effects of outpatient psychotherapy treatment and their association with patient, therapist and treatment characteristics. To reduce memory bias, 70 outpatient psychotherapy patients fulfilled the Inventory for the Assessment of Negative Effects of Psychotherapy (INEP). Data on patients' characteristics (sex, age, impairment), therapists' sex, and therpeutic alliance were collected. 84 % percent of patients reported at least one unwanted side effect (range 1-13; m=3.4; sd=3.43). Patients, age, number and kind of pretreatments, and the quality of the therapeutic alliance were associated with the frequency of unwanted negative effects. Unwanted side effects during outpatient psychotherapy are common phenomena and need careful attention in patient education and during the treatment itself.